This invention relates generally to apparatus and methods for mixing at least two substances, for example but not by way of limitation, dry cement and water. The invention relates more particularly, but not by way of limitation to a mixer incorporating a jet valve which provides increased mixing energy with which a cement slurry can be formed for use in an oil or gas well.
Well drilling and completion operations often require on-site mixing of various substances, such as cement slurries, acids and fracturing gels and weighting drilling fluids. In general, a mixing system includes a tub, pumps and various monitoring and control equipment. Cement slurries must be pumped into wellbores for a variety of reasons, such as for example securing casing in a wellbore. The mixture of cement to be used in a particular well typically is required to have certain characteristics which make the mixture, referred to as a cement slurry, suitable for the downhole environment where it is to be used. The desired type of cement slurry must be accurately mixed and produced at the well location so that it can be pumped into the wellbore.
Prior art apparatus for creating cement slurries include a jet mixer which typically sprays water under pressure into a venturi tube where bulk cement is added. The water and bulk cement combine to form a cement slurry which is conveyed into a tub prior to pumping the slurry down a wellbore. Another prior art mixer is shown in U.S. Pat. No. 5,046,855 (the ""855 patent), the details of which are incorporated herein by reference. The ""855 patent discloses a mixer with a flat orifice plate and a flat valve plate which can be utilized to regulate water flow. The valve and orifice plates are positioned horizontally in the mixer so that water must be falling downwardly, which is the same direction as the direction of flow of cement, when it engages the valve and orifice plates.
Another mixing apparatus is shown in U.S. Pat. No. 5,538,341 (the ""341 patent), the details of which are incorporated herein by reference. The apparatus shown therein discloses a mixing tube with a dry substance inlet, a mixed substances inlet and a liquid inlet. The patent discloses that a water metering valve is to be connected to the liquid upstream from the liquid inlet. Such prior continuous mixing systems work well and have served and continue to serve useful purposes. However, while the prior art apparatus and methods provide satisfactory results, there is always a need for mixing devices which can provide improved efficiency and improved mixing, and which more effectively utilize available mixing energy. The present invention provides such an apparatus.